I run semi-synthetic 10W40 and leave it in there for the full-monty 12,000 miles service interval.
I've used Selenia, Shell Helix, Castrol and even the Big H. green can stuff. When I fixed the head gasket I noticed all sorts of gum and varnish in there... nothing tooooo serious but it looked not very nice, so I nuked it with kerosene and then filled the beast up with Halfords 10W40 semi-synthetic for Diesel engines, as it's got more detergent in it. That flushed out a fair bit of post-rebuild black stuff and motor looks pretty clean in there still.
It's using 15W40 at the moment, as I just been down to Italia on the autostrada (engine oil touching 98C for extended periods, so I wanted as much viscosity as I dared).
Next change (next week) should be back to 10W40 as it's winter soon. Got half a can of Halfords green... so may use that, or I may spoil the beast and buy some synthetic Motul 6100 (?) for it.. as the syntho' stuff has more detergents in it than semi-synth. Need to find a 25 ltr drum though.. the 4ltr cans makes it an expensive hobby otherwise.
I don't want to change the oil more than 12,000 miles though... my engine wear is miniscule (even with 200,000m up) so apart from the gum/varnish issue, the 12,000 oil mileage isn't causing any problems.